Defects in the subsystems of a xerographic, electrophotographic or similar image forming system, such as a laser printer, digital copier or the like, may give rise to visible streaks in a printed image. Streaks are primarily one-dimensional defects in an image that run parallel to the process direction. Typical defects might arise from a non-uniform LED imager, contamination of the high voltage elements in a charger, scratches in the photoreceptor surface, etc. In a uniform patch of gray, streaks and bands may appear as a variation in the gray level. In general, “gray” refers to the intensity value of any single color separation layer, whether the toner is black, cyan, magenta, yellow or some other color.
One method of reducing such streaks is to design and manufacture the critical parameters of the marking engine subsystems to tight specifications. Often though, such precision manufacturing will prove to be cost prohibitive.
One technique to eliminate these streaks is to measure the cross process uniformity of a series of strips of different gray levels. Each strip is first averaged in the process direction to produce a profile for that gray level. A printer model is then constructed from the profiles, which is used to calculate a set of spatial tone reproduction curves (s-TRC's). The s-TRCs are a set of TRCs that capture variation in the cross-process direction and are used to compensate the streaking. If the printer has process direction banding, if the strip density depends on the photoreceptor position, or if the illumination of the sensor changes with time, then the mean strip profile will vary between strips. These errors are called process direction noise. This noise will lead to an imperfect measurement of the engine response curve (ERC), thus an error in the printer model and an imperfect compensation.
A tone reproduction curve (TRC) may be measured by printing patches of different bitmap area coverage. In some digital image processing applications, the reflectivity of a patch of gray is measured with a toner area coverage sensor. The manner of operation of the toner area coverage sensor is described in U.S. Pat. No. 4,553,033, which is incorporated herein by reference in its entirety. Toner area coverage sensors are typically designed with an illumination beam much larger than the halftone screen dimension. In addition, the toner area coverage sensors are typically designed to measure a fixed position and do not have the capability to move in the cross process direction. This large beam does not provide the resolution for the toner area coverage sensor to be useful as a sensor for the narrow streaks that may occur for poorly performing subsystems.
U.S. Pat. No. 6,760,056 by Klassen et al., incorporated herein by reference in its entirety, discloses one exemplary embodiment of a method for compensating for streaks by introducing a separate tone reproduction curve for each pixel column in the cross process direction. A compensation pattern is printed and then scanned to first measure the engine response curve at each spatial position and then detect and measure streaks. The tone reproduction curves for the pixel columns associated with the streak are then modified to compensate for the streak.